The primary aim of this study is to conduct epidemiological research on the association between drug use and HIV infection and transmission in female sex workers (FSWs) in Kaiyuan City, Yunnan Province, China. Chinese FSWs have been shown to have a high rate of HIV infection and are a potential bridge for HIV transmission to the general population. The proposed study extends our current epidemiological research (CIPRA CH 005), which has shown high prevalence of HIV infection, of other sexually transmitted infections (STIs), and of drug use in FSWs. Fifty-eight percent of HIV-infected FSWs are drug users. An integrated intervention targeting drug use and sexual risk behaviors among FSWs is greatly needed to curb the spread of HIV infection in this high-risk population. Prior to developing such an intervention, we need to understand more about how these women become HIV-infected and how drug use influences their sexual risk behavior. Our study will address the following new questions: (1) Do the women become FSWs in order to pay for their drug use, or do they become drug users after they become FSWs? (2) Is the primary risk of HIV infection in the FSWs from their sexual activity or from their (or their regular partners') injection drug use? (3) How many new HIV infections are diagnosed in a year, and are they among drug users or non-drug users? (4) How many women start or continue to use drugs over the course of a year? (5) Are drug-using HIV-infected FSWs more likely than those who are non-drug using to continue practicing commercial sex 1 year or more after learning their HIV status? (6) Do drug using FSWs enter drug treatment programs? We will establish two cohorts to answer these questions. Cohort 1 will include 300 drug-using and non-drug-using FSWs. Participants will complete sociodemographic, behavioral, drug use, and medical history questionnaires and medical examinations, and be tested for HIV and other STIs at month 0, 6, and 12 visits. Cohort 2 will include 100 HIV-infected FSWs to understand how drug-using and non-drug-using infected FSWs differ in their ongoing commercial sex work, risky behaviors, and utilization of HIV treatment and care. Both cohorts will provide detailed histories of drug use, commercial sex work, risky sexual behavior, and health care and drug treatment program utilization. The knowledge gained from this study will be used to develop an HIV transmission prevention intervention for FSWs in China.